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The flu is one of the most common infectious diseases caused by a virus. It is highly contagious and in the United States infects up to 20 percent of the population in some years. Of those infected, more than 200,000 people are hospitalized and 36,000 die due to the flu or its complications, according to the U.S. Centers for Disease Control and Prevention. Most outbreaks of the disease are seasonal, but in tropical areas they may occur throughout the year.

While the body is able to develop immunity to many viral diseases after a single infection, the flu causes outbreaks nearly every year. This occurs because proteins along the surface of the flu virus change over time. Antibodies produced by the immune system initially do not recognize the changed virus and an infection may occur.

Anyone exposed to the flu virus can become infected with the flu. In most people, the disease causes fever, muscle pain and headache. These symptoms typically pass within days but some people may develop severe complications, such as pneumonia or Reye Syndrome, a deadly disorder associated with the use of aspirin in children with viral infections. Severe symptoms related to the flu are most likely to occur in adults older than 65 and children who are 2 years of age or younger. Certain chronic medical conditions, such as malnutrition and asthma, are also associated with an increased risk of severe flu symptoms and complications.

Most cases of the flu are diagnosed from its symptoms. The virus can be confirmed and identified by culturing it. When the disease is suspected outside of periods of outbreaks, additional methods may be used. Treatment often consists of supportive therapy such as plenty of rest and fluids to prevent dehydration. For severe cases and in those who are likely to develop complications, antiviral medications may be prescribed. In addition, antibiotics may be recommended for patients who develop bacterial complications including pneumonia and ear infections, although antibiotics have no effect on the original viral flu infection.

The flu may be prevented in many people through annual vaccination. These vaccines may be injected or inhaled and are available to anyone older than six months. Patients who are sensitive to eggs or egg products may have an allergic reaction to the vaccine and are not advised to receive it.

Flu viruses are also found in other species, such as bird flu. These viruses rarely cause disease in humans, but have the potential to trade genetic material with viruses that cause seasonal flu in people. This may result in a virus against which there is little immunity in the population and can cause disease on a scale similar to that of the 1918 flu pandemic.